Abstract The accumulation of Myeloid-derived suppressor cells (MDSC) in individuals with cancer has emerged as a major mechanism of evasion of anti-tumor immunity and a primary obstacle to the development of efficient cancer immunotherapies. Despite their undeniable relevance in tumor-induced immune suppression, there are no current approaches to effectively block the immunosuppressive activity of MDSC in patients with cancer. Thus, novel therapeutic strategies to inhibit MDSC are urgently needed. Throughout the proposed research, we aim to determine the mechanisms by which the antibody-based blockade of the Notch ligands Jagged1-2 in tumor-bearing hosts functionally transforms MDSC into myeloid subsets that prime anti-tumor T cell responses. This is supported by crucial supporting results showing that treatment of tumor-bearing hosts with a humanized blocking antibody that recognizes the human and murine Jagged1-2 (CTX014) significantly delayed tumor growth and transformed MDSC into cellular populations that promoted the infiltration of reactive CD8+ T cells into tumors and enhanced the efficacy of T cell-based immunotherapy. Therefore, we hypothesize that: 1) The expression of Jagged1-2 in cancer cells and/or tumor-infiltrating MDSC plays a central role in the suppression of protective T cell immunity in tumors; and 2) Treatment of tumor-bearing mice with anti-Jagged1-2 therapy functionally reprograms MDSC, overcomes tumor-related T cell suppression, and increases the efficacy of promising cancer immunotherapies. To test these postulates, we proposed the following Specific Aims: 1) Determine the role of cancer cell-Jagged1-2 in the immunosuppressive activity induced by MDSC in tumor- bearing hosts; 2) Elucidate the mechanisms leading to the upregulation of Jagged ligands in tumor-MDSC and understand the endogenous effects of MDSC-expressed Jagged1 in tumor-induced tolerance; 3) Test the prediction that the combined inhibition of Jagged in cancer cells and MDSC overcomes tumor-induced T cell suppression and enhances the efficacy of various forms of immunotherapy. Completion of this highly innovative and translational research will elucidate the role of Jagged1 and 2 as primary mediators for the T cell dysfunction occurring in tumors and pave the way for the development of a novel therapeutic approach to functionally reprogram MDSC in patients with cancer, which is expected to prevent and/or reverse tumor- induced T cell tolerance and boost the efficacy of promising forms of cancer immunotherapy.